EP-B-0176508 discloses a design for a gas-fired autoclave which is useful e.g. in the production of articles from fiber/resin mixtures and heat treatment of workpieces in the glass, automotive and aerospace industries and which nowadays typically have working temperatures of up to 450° C. and working pressures of up to 68 Bar. Autoclaves for use in curing composites or heat-treating glass articles might typically have a length of 3-4 meters, a diameter of 1-3 meters and a volume of 10-20 m3. For use in the automotive industry e.g. for heat treating the chassis of a racing car an autoclave may typically have a diameter of about 2.75 meters with a length of about 4.5 meters and an internal volume of about 25 m3. For use in the heat treatment of aerospace components, an autoclave might typically have a diameter of about 4.25 meters, a diameter of about 12 meters and a volume of about 170 m3.
As shown in FIG. 1, a typical prior art autoclave is based on a pressure vessel that has a length of about 3.7 meters (12 feet) and a diameter of about 1.5 meters (5 feet), the vessel having a body 10 and a loading door 12. Vacuum lines 14 are provided for connection to the mold side of a mold tool (not shown) that is covered by a flexible diaphragm with a workpiece to be molded located between the tool and the diaphragm. The tool is connectable through valve 18 to vacuum and through valve 20 to air. Valve 22 can be operated to admit air through pressure lines 16 to the interior of the pressure vessel. Heating is by exposed radiant tubes 24 that run up and down the length of the pressure vessel. The entry to each tube is provided with a gas-fired heater 34 and the discharge end of each tube is provided with an impeller 36 by which a negative pressure is produced towards the discharge end and a flow of flue gas is maintained through the tube. A motor 38 mounted on the tank end wall drives a radial flow impeller 40 to produce a re-circulating flow of the gas within the pressure vessel. Thermocouples 42 through the tank wall 10 responsive to gas temperature are connected to a control unit 44 that is operatively connected to the various heaters to turn them off or on and maintain the gaseous atmosphere within the autoclave at ±1° of an intended value. The use of a variable speed impeller to enable the same tubes to be used for heating and for return to room temperature during the cooling part of the operating cycle is disclosed in EP-A-0333389. Autoclaves of other designs may be electrically heated, steam heated, oil-heated, hot air heated or gas radiant-heated, but up to now they have relied on an impeller in the end wall to produce a single generally axial pattern of re-circulating gas flow as indicated by the arrows in FIG. 1.
U.S. Pat. No. 6,240,333 (Lockheed-Martin) concerns the fabrication of composite parts in an autoclave. Lockheed-Martin explain that the F22 Raptor is an example of an aircraft made largely from composite materials formed with flexible graphite fibres, called a ply, that are impregnated with epoxy or BMI resins which harden when subjected to the application of heat. The uncured plies are placed on tools, each tool corresponding to a composite part of the Raptor. Thus, when the graphite resin mixture hardens over the tool, the composite part is formed with the proper shape. Lockheed-Martin go on to explain that a number of production techniques are available for forming composite parts. Again, using the Raptor as an example, once the plies are placed over the tool, a vacuum bag is used to hold the plies securely to the tool during curing of the resin. The vacuum bag forces the material to the tool and prevents the formation of bubbles and other material deformities. The tools are then placed in an autoclave for heating according to a schedule, adherence to which may be essential in order to avoid the production of defective parts.
Lockheed-Martin further explain that an autoclave operator must carefully distribute tools in the heating chamber of the autoclave to ensure that heating rate specifications are met, a typical autoclave being 15 metres (50 feet) long but nevertheless still being heated by blowing air with a large fan located at one end of the heating chamber. They identify a number of difficulties that this method of heating introduces into the production process, amongst others that if an autoclave operator adjusts heating rates to a lower level in order to avoid over-heating of a part, the autoclave will require a greater time to cure other parts, increasing the time required for the entire production run, and that if the parts are distributed improperly, the autoclave operator may have to violate the heating rate specifications for some of the tools, thus wasting the parts on those tools, in order to obtain useful parts from other tools. The solution suggested by Lockheed-Martin is to provide load distribution software for appropriate positioning of workpieces within a load to be introduced into the autoclave. The software includes a layout engine for determining the best layout of selected tools in an autoclave heating container depending upon (a) the particular tools selected, (b) the thermal performance of the tools and (c) the thermal characteristics of the autoclave, the layout engine generating the resulting pattern on a graphical user interface. The layout pattern is determined depending on:                Thermal response of the tools stored in a database.        Radial and axial variance in autoclave heating, the slow responding tools being laid out in regions of high heating and the fast responding tools being located in regions of low heating.        Uniform airflow around the load.        Feasibility of loading in the indicated pattern.However, Lockheed-Martin give no detailed directions about how a layout engine should be written and what calculations it should perform, particularly as regards uniformity of airflow.        